We need to keep in mind that using both SDSS and WISE data introduce selection bias to the SDSS-WISE quasar sample and also
bias to the color-redshift relationships, since it is biased toward quasars that are intrinsically bluer and were selected by SDSS
as quasars. These biases are difficult to avoid when the currently largest quasar sample based on SDSS is adopted. In addition, our requirement for quasars detected in all SDSS \(ugriz\)  bands and using only quasars with smaller magnitude
uncertainties in constructing the median color-redshift relations can introduce bias too, which also make the median color
bluer. Although we can treat the low S/N measurements with some statistical tools, we believe that using the relatively more accurate observational
data is still the most direct and efficient way to derive reliable color-redshift relationships.
Therefore, our derived color-redshift relations based on the SDSS-WISE quasars may not be applicable to the reddened quasars and optically obscured
Type 2 quasars. Reliable color-redshift relations of these special quasars will be obtained and compared with the current
results only
when large samples of them are available in the future. Currently, it is still unclear what the fractions of
reddened quasars and Type 2 quasars are in the total quasar population [1]}, [2]}, [3]}, [4]}. Some techniques, involving mid-infrared and near-infrared data, have been proposed to find the
obscured quasars and reddened quasars [5]}, [6]}. Similarly, we expect that the WISE data can also provide such helps in constructing a
more complete quasar sample.
